Multimodal Kelvin Probe Force Microscopy Investigations of a Photovoltaic WSe₂/MoS₂ Type-II Interface

Atomically thin transition-metal dichalcogenides (TMDC) have become a new platform for the development of next-generation optoelectronic and light-harvesting devices. Here, we report a Kelvin probe force microscopy (KPFM) investigation carried out on a type-II photovoltaic heterojunction based on WSe₂ monolayer flakes and a bilayer MoS₂ film stacked in vertical configuration on a Si/SiO₂ substrate. Band offset characterized by a significant interfacial dipole is pointed out at the WSe₂/MoS₂ vertical junction. The photocarrier generation process and phototransport are studied by applying a differential technique allowing to map directly two-dimensional images of the surface photovoltage (SPV) over the vertical heterojunctions (vHJ) and in its immediate vicinity. Differential SPV reveals the impact of chemical defects on the photocarrier generation and that negative charges diffuse in the MoS₂ a few hundreds of nanometers away from the vHJ. The analysis of the SPV data confirms unambiguously that light absorption results in the generation of free charge carriers that do not remain coulomb-bound at the type-II interface. A truly quantitative determination of the electron–hole (e–h) quasi-Fermi levels splitting (i.e., the open-circuit voltage) is achieved by measuring the differential vacuum-level shift over the WSe₂ flakes and the MoS₂ layer. The dependence of the energy-level splitting as a function of the optical power reveals that Shockley–Read–Hall processes significantly contribute to the interlayer recombination dynamics. Finally, a newly developed time-resolved mode of the KPFM is applied to map the SPV decay time constants. The time-resolved SPV images reveal the dynamics of delayed recombination processes originating from photocarriers trapping at the SiO₂/TMDC interfaces

Strain Superlattices and Macroscale Suspension of Graphene Induced by Corrugated Substrates

We investigate the organized formation of strain, ripples, and suspended features in macroscopic graphene sheets transferred onto corrugated substrates made of an ordered array of silica pillars with variable geometries. Depending on the pitch and sharpness of the corrugated array, graphene can conformally coat the surface, partially collapse, or lie fully suspended between pillars in a fakir-like fashion over tens of micrometers. With increasing pillar density, ripples in collapsed films display a transition from random oriented pleats emerging from pillars to organized domains of parallel ripples linking pillars, eventually leading to suspended tent-like features. Spatially resolved Raman spectroscopy, atomic force microscopy, and electronic microscopy reveal uniaxial strain domains in the transferred graphene, which are induced and controlled by the geometry. We propose a simple theoretical model to explain the structural transition between fully suspended and collapsed graphene. For the arrays of high density pillars, graphene membranes stay suspended over macroscopic distances with minimal interaction with the pillars’ apexes. It offers a platform to tailor stress in graphene layers and opens perspectives for electron transport and nanomechanical applications

Stability of the In-Plane Room Temperature van der Waals Ferromagnet Chromium Ditelluride and Its Conversion to Chromium-Interleaved CrTe₂ Compounds

Van der Waals magnetic materials are building blocks for novel kinds of spintronic devices and playgrounds for exploring collective magnetic phenomena down to the two-dimensional limit. Chromium–tellurium compounds are relevant in this perspective. In particular, the 1T phase of CrTe2 has been argued to have a Curie temperature above 300 K, a rare and desirable property in the class of lamellar materials, making it a candidate for practical applications. However, recent literature reveals a strong variability in the reported properties, including magnetic ones. Using electron microscopy, diffraction, and spectroscopy techniques, together with local and macroscopic magnetometry approaches, our work sheds new light on the structural, chemical, and magnetic properties of bulk 1T-CrTe2 exfoliated in the form of flakes having a thickness ranging from few to several tens of nanometers. We unambiguously establish that 1T-CrTe2 flakes are ferromagnetic above room temperature, have an in-plane easy axis of magnetization, and low coercivity, and we confirm that their Raman spectroscopy signatures are two modes: E2g (103.5 cm–1) and A1g (136.5 cm–1). We also prove that thermal annealing causes a phase transformation to monoclinic Cr5Te8 and, to a lesser extent, to trigonal Cr5Te8. In sharp contrast with 1T-CrTe2, none of these compounds have a Curie temperature above room temperature, and they both have perpendicular magnetic anisotropy. Our findings reconcile the apparently conflicting reports in the literature and open opportunities for phase-engineered magnetic properties